The present invention relates to a guide-bar device for automatic lathe, comprising a plurality of cylindrical cavities of different diameters provided for receiving and guiding bar stocks of different diameters and or profiles to be machined in the lathe, said cylindrical cavities being provided in a cylindrical body of circular or polygonal cross section having a length at least equal to the length of the bar stocks to be machined, all cylindrical cavities being parallel to the axis of the cylindrical body and at the same distance from said axis.
Such a guide-bar device is known from the patent EP 0 070 073 according to which the cylindrical body is formed by a plurality of longitudinal pieces of central profile and at least of a plurality of pieces of marginal profile assembled around the pieces of central profile, the periphery of each piece of central profile being provided with arcuate longitudinal grooves and the internal surface of each piece of marginal profile being provided with arcuate longitudinal grooves cooperating with the longitudinal grooves of the pieces of central profile for creating cylindrical cavities in the cylindrical body. Such a construction of the guide-bar device is relatively expensive because of the number and the variety of the pieces of central and marginal profile and of the complexity of the assembly of these pieces. Moreover, the cylindrical cavities of the cylindrical body comprise generally protective tubes driven in these cavities and forming then the proper guiding tubes. In this case, the indexing of the cylindrical body is not accurate because of the tolerances of concentricity of the tubes of different diameters provided in this cylindrical body, these tolerances having the effect that the centers of the tubes cannot be arranged exactly at the same distance from the axis of the cylindrical body and by the fact that the construction is made of many juxtaposed pieces which is prejudicial to the precision of indexing. The guiding tubes described above further comprise push-pistons provided for controlling the advance of the bars to be machined towards the headstock of the lathe. In this case, there is provided a radial play between the push-pistons and the internal face of the guiding tube for permitting a certain quantity of oil under pressure to pass from the rear part of the push-piston to the front part thereof, between the bar and the internal face of its guiding tube, in order to create a hydrodynamic bearing supporting the bar in rotation in its guiding tube. However, the passage of the oil through the mentioned radial play gives rise to an undesired loss of pressure.
One knows also from the published application EP 0 214 926 a guide-bar device for a single spindel lathe, comprising a unique guiding element of the bar to be machined. At the time of changing the diameter of the bar to be machined, it is necessary to replace the actual element by another element adapted to the diameter of the new bar which leads to foresee undesirable, removable guiding elements. Moreover, at the time of changing the guiding element, losses of the residual oil in the guiding tube may happen. As in the preceding case, the device comprises a push-piston and a loss of pressure of the oil is also present between the rear part and the front part of the push-piston. This loss is created by the relatively great length of the push-piston and the relatively small radial play between its outer diameter and the internal diameter of the guiding tube.